1. Field of the Invention
The present invention relates to an accelerometer and, more particularly, to a wide-band servo accelerometer with a flat leaf flexure.
2. Description of the Prior Art
Servo accelerometers (also known as force rebalance accelerometers) which include one or more electromagnet assemblies, which return the accelerometer to a null position in response to an acceleration or force input, are generally known in the art. Examples of such accelerometers are disclosed in U.S. Pat. Nos. 4,182,187; 4,250,757; 4,394,405; 4,399,700; 4,400,979; 4,441,366; 4,555,944; 4,555,945; 4,592,234; 4,620,442; 4,697,455; 4,726,228; 4,932,258; 4,944,184; 5,024,089; 5,085,079; 5,090,243; 5,097,172; 5,111,694; 5,182,949; 5,203,210; 5,212,984; and 5,220,831, all herein incorporated by reference. Such servo accelerometers normally include a proof mass, known to be formed from amorphous quartz, suspended by one or more flexures, which restrain the proof mass to a single degree of freedom, to enable the proof mass to deflect in response to forces or accelerations along a sensitive axis; generally perpendicular to the plane of the proof mass. At rest, the proof mass is normally suspended equidistantly between upper and lower excitation rings. Electrically conductive material forming pick-off capacitance plates is disposed on opposing sides of the proof mass to form capacitive elements with the excitation rings. As will be discussed in more detail below, the capacitive elements act as a position detector for the proof mass to provide a signal representative of the displacement of the proof mass from a null position. In particular, an acceleration or force applied along the sensitive axis causes the proof mass to deflect either upwardly or downwardly which, in turn, causes the distance between the pick-off capacitance plates and the upper and lower excitation rings to vary. This change of the distance between the pick-off capacitance plates and the upper and lower excitation rings causes a change in the capacitance of the capacitive elements. The difference in the capacitances of the capacitance elements is thus representative of the displacement of the proof mass along the sensitive axis. This displacement signal is applied to a torque to balance servo system (also known as a torquer or forcer) that includes one or more electromagnets which return the proof mass to its null or at rest position to further restrain the single degree of freedom of the proof mass. The magnitude of the drive currents applied to the electromagnets, in turn, is representative of the acceleration or force along the sensitive axis.
The proof mass or pendulum in such servo accelerometers is known to be suspended by either a pivot jewel or a flexure. For a relatively broad range of applications, flexure suspension is known to be preferred because of the many benefits over the pivot jewel suspension, including stepless bias response for accuracy in low vibration environments. Flexures also have no wear-out mode; are relatively less labor intensive to produce than pivot jewel suspensions; and are better suited to high volume process control production.
There are two known types of flexure suspension; circular arc flexures and flat leaf flexures. For a given angular stiffness, circular arc flexures are extremely short and have relatively low transverse compliance which reduces suspension errors. However, such circular arc flexures are relatively thin and fragile unless formed from relatively high strength materials which makes such flexures relatively expensive to produce. Flat leaf flexures, on the other hand, are relatively long and thick, and offer a geometry that is compatible with batch process etching techniques. The flat leaf flexures are rugged and tolerant, even when fabricated from relatively low strength materials due to their thickness. However, such flat leaf flexures are prone to suspension errors due to their relatively large transverse compliance, known as "S" bending. Although the ruggedness of flat leaf flexures and their compatibility with batch etching techniques makes the flat leaf flexures the choice for quartz accelerometers, the relatively large transverse compliance can result in cross-coupling errors, as well as mechanical limits to the servo acceleration which heretofore has made such flat leaf flexures undesirable.